Common types of semiconductor device are often made by many companies with the same type number, so that purchasers might quite justifiably assume that they are identical. In fact there are several possibilities. The devices may be made under licence using the same mask set and processes, or they may be completely different inside, or they may even be illegal copies. If they are internally different, the major characteristics may be the same but secondary characteristics are likely to be different, and this may be important in some applications. The transistors below have the same type number, but are very different inside the package.

Same transistor type number but different makes

We have recently compared five supposedly very similar types (but not identical type numbers) of Triac. These had chip sizes of 2.2, 3.0 and 4.0 mm square, and one or two wires of various sizes connected to the main terminal. In one case there was a copper ribbon rather than an aluminium wire. The largest chip with the copper ribbon, did not have the highest rating on the data sheets. One naturally wonders how the manufacturers determine the rating and whether the techniques are likely to be realistic for the current use.

Three nominally similar Triacs at the same magnification

The popular '741' operational amplifier comes in more than one internal format. They all have the same type number and conform in their major characteristics. The offset null controls on one group were far more susceptible to damage than on another. After opening them we found that the layout of the tracks and the way they crossed other components varied between the types.

Previously ERA had examined some modules from satellites. Some were thick film hybrids formed from discrete active devices and printed resistors but some were ICs. They all had the same part number but could not be distinguished from the outside.

All these internal differences may be critical to reliable operation, particularly in power devices. ERA can quickly open packages and compare chips, and can selectively remove protective coating layers by plasma etching to compare fine circuitry details using the SEM. The techniques leave the fine bond wires intact and so devices remain fully functional.